Hi Bob and guys.

John Payson would be offered this.

movfw   DATAIN          ;abcd efgh > abcd hgfe
btfsc   DATAIN,0        ;exchange e and h
xorlw   0x09
btfsc   DATAIN,3
xorlw   0x09
btfsc   DATAIN,1        ;exchange f anf g
xorlw   0x06
btfsc   DATAIN,2
xorlw   0x06

1 clock less, result in W as well.

WBR Dmitry.



Bob Ammerman wrote:
>
> a simple technique, perhaps not the best
>
> movf    DATAIN,W        ; Get the data
> andlw  0xF0                    ; Keep 4 high bits
>
> btfsc    DATAIN,0          ; Low bit set?
> iorlw    1 << 3                 ; Put in new place
>
> btfsc    DATAIN,1
> iorlw    1 << 2
>
> btfsc    DATAIN,2
> iorlw    1 << 1
>
> btfsc    DATAIN,3
> iorlw    1 << 0
>
> result is left in W
>
> Bob  Ammerman
> RAm Systems
>
> ----- Original Message -----
> From: "Pic Dude" <picdude@AVN-TECH.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Friday, March 08, 2002 7:29 PM
> Subject: Bit operation quiz...
>
> Anyone up for a pop quiz?...
>
> I have all Port C outputs going to 8 LED's.  To save PCB space, I can
> re-route
> the 4 lowest lines to the LED's, but the last 4 bits get reversed.  What's
> the fastest
> way, in code, to reverse just those 4 lowest bits, and leave the 4 highest
> bits
> untouched?  Fastest = smallest # of steps.
>
> Assume input = ABCDEFGH, then output should be ABCDHGFE.
>
> So far I've come up with this (untested), but it's seems like a very
> "sledgehammer" way to do it.  There's gotta be something slicker.  I
> remember learning about the XOR swap trick many years ago, and
> thought it was the coolest thing ever (at the time).  Any thing like
> that exist for doing this partial reverse?
>
> movf    DATAIN,TEMP
> rrf    TEMP,F
> movf    TEMP,W
> andlw    H'02'
> movwf    DATAOUT    ; Bit 1 done
> rrf    TEMP,F
> rrf    TEMP,W
> andlw    H'01'
> andwf    DATAOUT,F    ; Bit 0 done
> movf    DATAIN,TEMP
> rlf    TEMP,F
> movf    TEMP,W
> andlw    H'04'
> andwf    DATAOUT,F    ; Bit 2 done
> rlf    TEMP,F
> rlf    TEMP,W
> andlw    H'08'
> andwf    DATAOUT,F    ; Bit 3 done
> movf    DATAIN,W
> andlw    H'F0'        ; Strip out low nibble
> andwf    DATAOUT,F    ; Merge with result
>
> Cheers,
> -Neil.
>
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